1. Field of the Invention
The present invention relates generally to precision optical inspection methods and apparatus, and more particularly to a method and apparatus for performing microscopic inspection and measurement of integrated circuit wafer geometry using interference microscopy in combination with electronic image processing.
2. Discussion of the Prior Art
It has long been desired that means be provided to inspect and measure the characteristics of microminiature surfaces such as those formed in integrated circuit wafers. One such characteristic of interest is the line widths of the various traces produced on a wafer surface during IC device manufacture.
One prior art technique for integrated circuit metrology includes the use of an ordinary microscope with some form of electronic detector positioned at the image plane. For example, video cameras, scanning slits (see U.S. Pat. No. 4,373,817), shearing systems and linear arrays, have all been used as detectors with ordinary microscopes. However, the capability of the ordinary microscope is limited in that it can only measure the intensity of the optical wave amplitude and cannot measure the complex phase of the amplitude. As a consequence, the three-dimensional nature of integrated circuit surfaces makes use of the classical microscope impractical for precision surface inspections and measurements of this type.
Other prior art techniques have used confocal laser scanning microscopes to obtain three dimensional data relating to integrated circuit surfaces. A rather thorough treatment of the subject may be found in T. Wilson and C. Shepard (1984), Theory and Practice of Scanning Optical Microscopy, Academic Press.
Aside from the complexity and relatively high cost associated with the use of confocal laser devices and techniques, the fact that such techniques use monochromatic light makes them subject to inaccuracies caused by destructive interference for certain thicknesses of transparent films often found in semiconductor devices.